Wearable heart rate sensors may typically employ photoplethysmography (PPG) techniques in which one or more light emitting diodes (LEDs) illuminate the skin of the wearer and changes in light reflected from the skin are measured as a function of pulsatile blood flow. While such an approach may be suitable under certain circumstances, there remains considerable room for improvement. For example, driving the LEDs may involve a relatively large amount of power consumption, which may have a negative impact on battery life, particularly in wearable sensors. Moreover, the accuracy of the reflected light in measuring pulsatile blood flow may be dependent on skin color as well as muscle tissue perfusion.